About This Article
In this STAN, we discuss items of interest to those who are preparing Phase I proposals for Cycle 33, provide updates on recent instrument calibration efforts, and share information on updated contact information for the STScI Help Desk.
Contents
End calloutPolicy Change on the Maximum Usage of Lifetime per Observing Program
Over the past few years, usage of the COS FUV detector has exceeded historical levels and increased the risk of degraded performance before 2030. To ensure that COS and its capabilities remain available into the 2030s, STScI is implementing a new policy for Cycle 33 aimed at preserving the COS FUV detector. The policy caps the usage to 2% of the lifetime at a Lifetime Position (LP) for an observing program. Additionally, lifetime usage beyond 1% for a program should be justified in the Phase I to address whether other modes with lower lifetime impact could be used and why observations could not be obtained with STIS. Furthermore, since Cycle 32, users are not allowed to obtain a signal-to-noise ratio (SNR) on a single target beyond that allowed by fixed pattern noise for a given mode.
STScI will require users to provide an estimate of their lifetime usage for given modes in their Phase I justification. All COS users should determine from the table below the approximate maximum number of targets observable under the lifetime usage cap of 2% for a given SNR and instrumental mode. If the number is within a factor of two of the limit (i.e., beyond 1% lifetime usage), users should perform a more careful calculation and consider optimizing the operational set-up to bring the lifetime usage down prior to submission (contact scientists of such approved programs will work with the PIs to minimize the lifetime usage). Lifetime usage beyond 1% (but below 2%) should be justified in the Phase I. The details of the policy and how to estimate lifetime usage are given below.
The COS bright-object safety screening limits (Section 10.2 of the IHB) for the FUV were chosen to protect the detector against overlight events that can occur over short time periods. However, very bright continuum sources, emission lines, or moderately bright objects observed for long periods of time can still deplete a significant percentage of lifetime for a given detector LP without violating the screening limits. Approximately 27,000 counts can fall on a pixel before the reduced gain at that location results in significant flux loss. FUV observational programs must keep the estimated total counts in the brightest pixel integrated over different targets for a given lifetime position under 2% of a pixel's lifetime, or 540 counts in a given pixel. Users in these situations should carefully consider whether sacrificing wavelength coverage by turning off segments might help keep their science under the lifetime caps. Note also that the 2% restriction refers to a single lifetime position. As of Cycle 33, LP7 will become the default for G130M 1055, 1096, 1222, and 1291 cenwaves, with G130M cenwaves greater than 1291 remaining at LP5. G160M cenwaves are planned to be at LP10, and G140L cenwaves are currently at LP3.
How to Determine the Lifetime Usage of Your Observation
Proposers will need to evaluate whether their observations exceed this limit if their total targets are within a factor of two of the numbers in Table 1. Users can do one calculation for their highest SNR target and apply it to all of their targets; if they still exceed the cap they will have to do more detailed calculations. The procedure is as follows:
- Perform an ETC Calculation: Run a COS ETC calculation for the science target and determine the brightest pixel count rate for each segment. If observations include airglow lines such as geocoronal Lyman-alpha or OI, use the "No airglow" option from Question 5c of the ETC to estimate the brightest pixel’s count rate. If unsure, the airglow feature can be turned off regardless of whether it is covered.
- Multiply by the Total Exposure Time: This assessment depends on whether the brightest pixel is dominated by a continuum or an emission line:
- Continuum-Dominated Observations: The total exposure time per grating should be used, irrespective of the number of FP-POS settings. Therefore, multiply by the total exposure time per grating per visit
- Emission Line-Dominated Observations: The total exposure time per FP-POS should be considered since the emission line will land on different regions of the detector at each FP-POS setting. Therefore, multiply by the exposure time per FP-POS per visit.
- Sum Across All Visits
- Check Against the 540-Count Limit: If the total exceeds 540 counts at a given LP, adjustments will be necessary, such as using a different mode, turning off a segment, reducing exposure time, or moving some observations to STIS.
Example Calculations
Example 1: Continuum Source
A bright A3V star is observed with G160M for a total exposure time of 11,590 s over five visits. ETC results show peak counts/s/pixel of 0.128 for Segment B.
Calculation: Since this exceeds the 540-count limit by a factor of 2.75, a less sensitive mode would be required, such as STIS/E140M. The ETC calculation for this example is: COS.sp.1895424
Example 2: Emission Line Source
Epsilon Eridani is observed with G130M/1222 for 12,000 s, using 4 FP-POS. ETC results indicate peak counts/s of 0.071 for Segment B.
Calculation: Since the total per FP-POS is below 540 counts, the observation remains within limits. Additionally, the total per FP-POS is below 270 counts, so this observation will not trigger a review or optimization. The ETC calculation for this example is: COS.sp.1923915
Example 3: Optimizing a program for a large number of targets
A program plans to observe 40 WDs at SNR=27 with G130M/1291. Looking at the table above, the estimated allowed number of targets in this mode at that SNR is only 22. The program could either move to G130M/1222 or consider moving some of the observations to STIS. The proposing team should still perform detailed ETC calculations to confirm that they will stay under the 2% cap and justify any lifetime usage beyond 1%. An example ETC calculation for G130M/1291 is: COS.sp.1954076 and for G130M/1222 is: COS.sp.1954080
For questions or assistance with estimating detector impact, please contact the HST Help Desk.
Changes in Overheads for FUV Spectroscopy with the G130M Grating at Lifetime Position 7 and G160M Grating at Lifetime Position 10
At the beginning of Cycle 33, LP7 will become the default for G130M spectroscopy using 1055, 1096, 1222, and 1291 cenwaves, with G130M cenwaves greater than 1291 remaining at LP5. Similarly, LP10 will become the default for G160M spectroscopy for all cenwaves. Similar to observations at LP6, G130M observations at LP7 will experience increased wavelength calibration overheads due to the introduction of SPLIT Wavecals. On the other hand, G160M observations at LP10 will no longer require SPLIT Wavecals. The Cycle 33 COS Instrument Handbook discusses the wavelength calibration overheads at LP6 due to the inclusion of SPLIT Wavecals. Scientists proposing the use of the G130M grating with cenwaves 1055, 1096, 1222, or 1291 in Cycle 33 are strongly encouraged to read this material regarding the overheads incurred at LP7.
Continued Enforcement of the COS2025 Rules for the G130M Grating
The COS2025 rules, designed to extend the lifetime of the COS FUV detector, are of particular interest to scientists who want the full two-segment wavelength coverage offered by the G130M grating or who want to observe Ly α in zero-redshift targets. The COS2025 rules will continue to apply in Cycle 33. The rules limit the locations where damaging Ly α airglow can fall on detector segment FUVB: 1) G130M cenwaves greater than 1291 cannot be used when segment B is on, and 2) G130M/1291 is limited to two FP-POS when both segments are on. Please see the COS2025 page for further details. A consequence of these rules is that, for zero-redshift astronomical targets, Ly α falls on gain-sagged regions of the detector at the default LP7. Users who wish to observe Ly α in such targets should instead use LP3. This option must be requested and justified in the Phase I proposal.
Special Requirements Must Be Justified in the Phase I Proposal
All special requirements must be requested and justified in the "Special Requirements" section of Phase I proposals. A list of such requirements appears in the Call for Proposals. For COS, the most common requirements are explicit scheduling requirements (e.g., for planetary transits), waivers of the FP-POS requirements, requests to use non-default lifetime positions, and ORIENT constraints.
ORIENT constraints are commonly used for extended sources (i.e., sources with FWHM > 0.6"). Observing these at different position angles can result in changes in the cross-dispersion and dispersion profiles on the COS detector. Such changes can lead to differences in spectral resolution and flux calibration that may prove problematic when combining spectra across visits or performing science on separate spectra taken at different position angles. This can be mitigated by implementing an ORIENT constraint across visits within APT. As with other special requirements, these must be requested and justified in the Phase I proposal.
Delivery of a new SPOTTAB
On March 10, 2025, the COS team released a new Spot Position Table (SPOTTAB) to mask out pixels on the FUV detector that are affected by spurious counts added by hot spots. Most hot spots are transient and rarely affect science data; however, they appear at random times and locations and can sometimes fall in the extraction zone of a spectrum, in the region where the background is calculated, or where a wavelength calibration lamp spectrum projects. These can sometimes result in artificial emission lines appearing in the reduced spectrum, an over-subtraction of background in the affected columns, and/or a failure in CalCOS when determining the wavelength calibration. This update includes new hot spots that have appeared since September 2022, and also fills in previously unknown hot spots and active time frames before 2022 that have been identified by a new, automated hot spot monitor.
STScI Help Desk Updates
We want to inform users that the STScI Help Desk email address (help[at]stsci.edu) will be deactivated by the end of this year. Moving forward, any help desk inquiries that you may have are to be relayed through the existing ServiceNow web portals for the relevant mission/division:
- HST: https://stsci.service-now.com/hst or hsthelp.stsci.edu
- JWST: https://stsci.service-now.com/jwst or jwsthelp.stsci.edu
- Roman: https://stsci.service-now.com/roman or romanhelp.stsci.edu
- OPO: https://stsci.service-now.com/opo or opohelp.stsci.edu
- MAST: https://stsci.service-now.com/mast or masthelp.stsci.edu
- ITSD (internal): https://stsci.service-now.com/stars or stars.stsci.edu
STScI has used the ServiceNow platform for the help desk since early 2017, but we have kept the email address active as a courtesy to the user base. These portals offer announcements, knowledge base articles that address frequently asked questions, options to submit your questions directly to the appropriate experts, and access to your support history. A free MyST account is required to use these services. For any issues accessing ServiceNow, please contact helpsn[at]stsci.edu.